Typically, the radiation pattern of a single element antenna is relatively wide and the gain (directivity) is relatively low. High gain performance can be achieved by constructing the antenna with a plurality of individual antenna elements in a geometrical and electrical array. These array antennas (or simply arrays) are typically used for applications requiring a narrow beamwidth high-gain pattern (i.e., low energy in the beam side lobes) and the ability to scan over a relatively wide azimuth region. Low side-lobe antennas are especially advantageous for satellite communications and scanning radars.
The individual antenna elements in the array are usually identical, although this is not necessarily required, and may comprise any antenna type, e.g., a wire antenna, dipole, patch or a horn aperture. The spacing of the elements is typically periodic. The composite radiation pattern of an array antenna array is determined by the vector addition of the electric and magnetic fields radiated by the individual elements. To provide a directive array antenna radiation pattern, the elemental fields add constructively in the desired direction and add destructively in those directions where no signal is desired. Also, the array antenna can be scanned over an angular arc by simply controlling the phase and/or amplitude of the signal input to each element. By contrast, scanning a parabolic dish antenna requires drive motors to physically move the dish through the desired scan angle.
Assuming the array antenna comprises identical antenna elements, there are five conventional array parameters that can be varied to achieve the desired antenna performance: the geometrical shape or configuration of the array antenna (e.g., linear, circular, rectangular, spherical), the relative displacement between the array elements, the excitation signal amplitude and phase that drives the elements and the radiation pattern of the individual elements.
Array antennas can be constructed in many different geometrical shapes. The most elementary shape is a simple linear array where the antenna elements lie along a straight line. A planar array is bounded by a closed curve; circular and rectangular are the most common planar array shapes. In a conformal array the elements and the substrate to which they are attached are made to conform to the surface of a structure, such as the skin of an aircraft.
However, array antennas are not without disadvantages. Each element is fed by a complex feed network of electronic components, but close element spacing (typically a half wavelength) requires a small pitch feed network. Squeezing the feed network into the small space between the elements presents difficult design and manufacturing challenges, resulting in an expensive feed network, and expensive, miniaturized element-level electronics (often referred to as element modules). The spacing problem is exacerbated at shorter operational wavelengths, i.e., at higher frequencies. Bandwidth limitations and mutual coupling between closely-spaced elements and their feeds also present disadvantages. It is also difficult to provide dual or multi-beam operation within an array antenna due to these various antenna element spacing issues.
In addition to forming an array antenna from individual elements, the antenna can be formed from a plurality of individual sub-arrays (also referred to as sub-array lattices or sub-array grids), where each sub-array further comprises a plurality of individual antenna elements arranged in a geometrical pattern. The individual sub-arrays are tessellated to form the array antenna. Four different sub-array grid configurations are commonly used and described below.
The periodic sub-array lattice comprises a plurality of equally-spaced elements arranged in the form of a polygon, such as a rectangle or an equilateral triangle. The triangle offers a higher packing density for the array antenna, as the sub-array triangles can be oriented to form a honeycomb pattern, and the effective per-element spacing is smaller. The element periodicity (i.e., the distance between individual elements of the sub-array) is established to produce the desired antenna operational characteristics, but as discussed above, closely-spaced elements require a closely-spaced and expensive feed network and array electronics.
The total scan angle and usable bandwidth for the periodic sub-array are limited by the presence of grating lobes in the radiation pattern. These grating lobes, which are major lobes in the radiation pattern with an intensity about equal to the main lobe, are especially prevalent at higher frequencies, such as X-band and Ku-band frequencies. Operation at lower frequency, such as UHF, L-band and S-band, have also been found to produce grating lobes in certain antenna arrays. Notwithstanding the grating lobes, the periodic array has a relatively high array efficiency as the antenna elements are efficiently dispersed through out the entire array antenna aperture.
A random sub-array, where the sub-array elements are randomly spaced with respect to each other, can reduce the grating lobes in the radiation pattern of the array antenna. The sub-array element spacing can be constrained so as not to exceed a given value (for example, a half-wavelength) or can be unconstrained. However, optimal element spacing for the random sub-array has not been determined and is not amenable to a closed form solution. Also, if the average spacing is permitted to exceed about a half wavelength at the operating frequency, performance of the array antenna is severely degraded. To form the array antenna, the random sub-arrays can be randomly positioned or the sub-arrays can be arranged in the shape of a polygon.
Any periodic sub-array can be thinned, i.e., elements randomly removed to reduce the side lobe energy, and to a lesser extent, the grating lobe effects. However, the thinning process has not been optimized nor quantified to produce predictable radiation patterns. As a result, considerable design effort is required for each specific application in which the thinning process is employed.
A plurality of ring sub-arrays (i.e., a series of concentric rings) can be used to form a main array antenna by spacing the sub-arrays either periodically or aperiodically. Also, the number of elements in each ring sub-array can be varied. For example, in addition to a central element, an inner sub-array ring can include 7 elements, surrounded by a second ring comprising 13 elements and further surrounded by a third ring comprising 19 elements. It has been determined that the ring is near optimal for grating lobe suppression when the number of elements in each sub-array ring is a prime number. Although an array antenna formed of ring sub-arrays reduces the grating lobes, there is no closed form solution for constructing the array. Like the random and thinned sub-arrays, each design application must be optimized by trial and error. Such an antenna array is disclosed and claimed in the commonly owned patent entitled, “Phased Array Antenna Using Aperiodic Lattice Formed of Aperiodic Subarray Lattices,” bearing issued U.S. Pat. No. 6,456,244, which is incorporated herein by reference and from which the present application is a continuation-on-part.
A high gain array antenna with wide angular coverage, is typically comprised of a plurality of panels, where each panel further comprises a plurality of sub-arrays. Each panel provides radiation coverage over a different spatial sector. For example, panels of sub-arrays can be configured on a pyramidal structure for providing hemispherical coverage.